The water in flowing through the suction opening, however, will partake of. the rotary motion of the impeller and will approach the point E, Fig. 10, at some unknown angle. The correct entrance diagram will then appear like that shown in the latter figure. As the angle is not known the angle of the vane is selected to suit the shape
of the vane and by experience. In any case the vane must be curved backward at point E.
The development of the fundamental equation is based on the ideal entrance diagram.
Multi-stage Centrifugal Pumps. For higher heads* two or more impellers may be arranged in series, the first impeller taking suction
from the well and discharging the FIG. 10.—Centrifugal-pump velocity diagram water ^Q the suction of the second with actual entrance diagram •
impeller, etc. The impellers are
mounted on the same shaft and built into one casing, designed to form suitable channels.
In order to reduce the leakage of water from the discharge to the suction side of the impeller wearing rings are used. Preferably they are made separate from the impeller so that they may be renewed if worn. They also serve to balance the outer part of the impeller, both sides of which will be subject to the same pressure. The H
Capacity in Gal.P.Min.
FIG. 11.—Head-capacity, power and efficiency curves for centrifugal pumps.
inner part of the impeller is balanced by coring holes through the web near the hub so that the leakage may return to the suction and prevent the pressure on the back from building up. This equalizing is not perfect and, therefore, a thrust bearing must be provided to taxe care of the unbalanced axial thrust.
By the use of double-suction impellers almost perfect balancing is accomplished. The design of a single-stage double-suction impeller pump is simple, see Fig. 12. Here the suction branches out so as to embrace the casting, and the water enters from both ends and is discharged in the center. Multi-stage pumps with double-